1. Field of the Invention
This invention relates to a circuit arrangement including a number m of converters fed from an a.c. voltage network. In particular it concerns a circuit arrangement comprising an odd number m of direct converters which are arranged in a symmetrical m-phase Y configuration and which are connected to an m-phase symmetrical load such that there is no direct connection between the neutral of the converters and the neutral of the load, the conductor voltages of the converters at the load possessing a given periodic waveform and together forming a symmetrical system.
2. Description of the Prior Art
The above-described type of circuit arrangement is used, for example, to feed rotary field machines or to feed land or board networks. The particular converters used in the circuit arrangement, typically, might be intermediate link converters, pulse inverters or direct converters. Such converters are equipped, in particular, with controllable semiconductor valves, usually thyristors. In a direct converter, generally all output conductors are connected with all input conductors via anti-parallel connected valves. The converting is brought about by a special ignition sequence of the valves. Normally, the output voltage is composed primarily of sections of the input voltage. Therefore, the output voltage is, generally, of a lower frequency than the input voltage.
In the prior art, it is customary to feed three reversible converters from a common a.c. voltage network and to modulate them with voltages which are offset in relation to each other by 120 electrical degrees. One thus obtains a three-phase current system at the output, which can be used, for example, to feed a three-phase current machine. Such a circuit arrangement can be employed also as power supply of land or board networks. When using a symmetrical three-phase load, the line current in all three output conductors is normally regulated in sine-shaped waveform. For this purpose three separate control loops are provided in the circuit arrangement which adjust the control signals of the three direct converters as a function of the deviation between the measured actual values of their line currents and given nominal values.
It is the nature of a direct converter that the network side power factor is relatively small when the direct converter is not fully modulated, because, due to the gating control of the direct converter, its reactive power requirement is high. By the "network-side power factor" it is meant that the cosine of the phase angle existing between the fundamental frequency of the feeding network voltage and the fundamental frequency of the feeding network current. Generally, one tries to make the network-side power factor of a converter fed from an a.c. voltage network as great as possible, thereby keeping the reactive power requirement of the converter to a minimum. Additionally, one also tries at the same time to reduce the voltage load of the valves at constant transmission efficiency. In other words, at a given input voltage the transmitted power should be as great as possible at any degree of modulation.
The object of the present invention is to increase the power transmitted from the m converters to the load in the above-mentioned circuit arrangement.